iv. The flight of the shell being regular, allowance can

be made for any deviation observed and thus increased accuracy may be obtained.

=100.= _Rifling._

1. The “_system of rifling_” is the term applied to the method adopted in any particular type of rifled gun for giving rotation to the shell.

2. The aim of each system is to produce accuracy of fire, but it is also essential that it should be simple, that it should not seriously weaken the durability of the gun, and that the shell should not be liable to jam in loading or firing. The “_twist of rifling_” by which is meant the distance measured in calibres, in which the grooves make one complete circuit of the bore, may be uniform, increasing or a combination of the two.

3. In the case of a uniform twist the distance in which the grooves make a complete circuit of the bore is the same wherever measured, but with an increasing twist the distance decreases as the muzzle is approached.

With a uniform twist the shell is compelled to rotate rapidly as soon as it begins to move, and thus a severe strain is caused both to the gun and shell.

With an increasing twist, this rotation is imparted more gradually, thus relieving the strain, but at the same time causing more friction, and consequent loss of velocity. The shell, moreover, is not so well centred as with a uniform twist.

4. In designing a gun the twist has to be made to suit the shell which it is desired to employ, and its intended velocity. Generally speaking, a long shell with low velocity necessitates a rapid twist to make it steady in flight.

=101=. _Centring._

It is important that the shell when it leaves the bore should be centred, _i.e._, that the shell should rotate round its longer axis which should coincide with the prolongation of the axis of the gun at the moment it leaves the bore. Should this not be the case, the shell becomes unsteady and noisy in its flight, and the shooting will be irregular.

=102.= _Forces acting on a shell in the bore._

1. The velocity attained by a shell at the muzzle of a gun is due to the pressure of the gas during its passage through the bore. The more gradually this velocity is imparted to the shell the less will be the strain upon it and the gun. The object sought is to distribute, as far as possible, the pressure over the whole length of the bore and to obtain the maximum work from a given charge without undue strain on either gun or shell. Theoretically the last atom of the charge should be converted into gas as the shell leaves the muzzle.

FORCES ACTING ON A SHELL DURING FLIGHT.

=103.= _The resistance of the air._

1. The air consists of innumerable small particles through which a shell has to force its way. This produces a rapid loss of velocity; for instance, the velocity of a shell from the 18-pr. Q.F. gun, which at the muzzle is about 1,610 feet a second, is at 2,000 yards only about 1,030 feet a second, and at 6,000 yards about 740 feet a second.

2. The retardation due to the resistance of the air varies according to the weight and diameter of the shell. If, for instance, two shells of equal diameter, but of different weights, start with the same muzzle velocity, the heavier will lose its velocity more slowly and have the longer range, because it has the greater weight with which to overcome the resistance of the air. On the other hand, if two projectiles are of the same weight but of different diameters, the one with the smaller diameter will have the advantage, because it presents less surface to the resistance of the air.

3. The shape of the head also materially affects the question, for a shell with a blunt head is plainly not so suited for forcing its way through the air as one with a more pointed head. Thus the longer the shell (other things being equal), the greater will be its remaining velocity at any given range, and the greater will be its range for any given muzzle velocity; but other considerations limit its length, such as the strength of its walls, and the liability to turn over in flight.

4. The relationship between the weight and diameter of a shell, represented by the expression

W ------, (n d²)

is called the “_ballistic coefficient_” of the gun. The factor n includes sub-factors which vary with the density of the atmosphere, the shape of the head, the steadiness in flight, &c. The greater the ballistic coefficient the less rapidly does the projectile lose its velocity.

5. It has already been pointed out (Sec. =99=) that spin is necessary to the shell if elongated projectiles are to be used. The effect of the resistance of the air on a spinning shell is to deflect it in the same direction as the spin. With all service shell this spin or rotation is right handed, and consequently all shell deflect towards the right in the course of their flight. This deflection is called “_drift_.”

As this rotation is definite and diminishes very slightly during the flight of the shell, the amount of drift can be determined for each nature of gun by actual experiment and compensation made for it. A description of the method by which this is done will be found in Sec. 119.

=104=. _The force of gravity._

1. The force of gravity is the natural attraction which causes every unsupported body to fall towards the centre of the earth. Moreover, the longer a body is exposed to its influence the faster does it fall; thus a body falls,

About 16 feet in the 1st second ” 48 ” ” 2nd ” ” 80 ” ” 3rd ”

that is, the total drop at the end of any given second of time is proportionate to the square of the time.

Now a shell leaves the muzzle of a gun with a certain velocity, due to the forces which acted on it in the bore, and, if the effect of these forces and of the resistance of the air had alone to be taken into account, it would proceed in a straight line, but at a decreasing pace.

Gravity, however, comes into play and causes the shell to fall with a constantly increasing velocity. Thus in Fig. 12, supposing AB is the direction in which a shell starts, the distances it would travel in the first three seconds of its flight, if there were no gravity, might be represented by AC, CE, EG. Owing to gravity, however, it would at the end of the 1st second have dropped to some point D; at the end of the 2nd second it would have dropped four times as far, to a point F; while at the end of the 3rd second it would have fallen nine times as far, to a point H.

2. The force of gravity is to some extent counterbalanced by the air, which acts as a cushion and tends to support a body falling through it. In the case of a heavy body, such as a bullet or shell, the distances fallen are but slightly affected by the air.

Gravity is, however, appreciably counterbalanced by the “_planeing_” effect on an elongated projectile forced at considerable velocity through the air at high angles of elevation so that the value of “g” drops below 32 to about 26.

=105.= _Trajectory._

1. The result of the three forces acting on the shell--the force of projection tending to drive it forward in a straight line in prolongation of the axis, the force of gravity drawing it down from that line, and the resistance of the air tending to stop its progress in each successive instant of time, and also to deflect it to the right--is that it describes a curve, whether regarded from the side or from above. This curve is called the trajectory. (_See_ ADFHI, Fig. 12.)

2. The shell in consequence of its high velocity, and of the short time gravity has had to act upon it, falls at first very little, but this fall--and consequently the curve--increases very rapidly with the range; for example, at 100 yards the 18-pr. Q.F. shell if fired point blank would fall about 8½ inches, but at 500 yards its drop would be about 18¼ feet; at 1,000 yards, about 80 feet; at 2,000 yards, about 372 feet; at 3,000 yards, about 970 feet. The greatest height attained by the trajectory is roughly four times the square of the time of flight.

=106.= _Elevation._

1. It is evident from what has been said that a shell fired from a gun at a mark, S (Fig. 13) will not hit that mark, but will strike some point, D, below it. To allow for the fall, it is necessary to point the axis as much above the object to be hit as the shell would have fallen below it if the axis had been pointed straight at the mark. This act of tilting the gun so as to allow for the curve of the trajectory, is what is meant by the expression “_giving elevation_.”

2. The theory of giving elevation may be illustrated by reference to the flight of an 18-pr. Q.F. shell for the first 100 yards. This shell falls about 8½ inches below the line at which the projectile leaves the bore in passing over the first 100 yards of its flight. Assuming that S (Fig. 13) is 100 yards from the muzzle and that SD represents 8½ inches, the axis of the gun must be elevated so that, when produced, it would pass 8½ inches above S, viz., through F (Fig. 14), if an object at S is to be hit.

3. The method of giving elevation to the gun to enable the shell to reach a certain distance called the “_range_” should then be explained, and it should be shown how the elevation can be marked in yards on the range drum, tangent sight or clinometer.

4. The principle of the sighting can be shown by means of a rifle barrel placed on a tripod a few yards from a blackboard. If then aim be taken at some mark on the board with the leaf of the sight upright, and set to (say) 1,000 yards; and subsequently, without moving the barrel, the bore be looked through, it will be seen that the axis produced meets the blackboard considerably above the mark aimed at.

5. The effect on the shooting of the gun, if the wheels are not level, can be explained by similar means. A stick to represent the trunnions of the gun should be fastened across the barrel. It can then be shown that if the wheels of a gun are not level, or in other words if the axis of the trunnions is not horizontal, the gun (barrel) will not elevate in a vertical plane; the muzzle will move towards the lower side and the breech towards the higher, thus causing the shell to move towards the lower wheel.

6. The effect on the sight, if the axis of the trunnions is not level, can also be demonstrated. The more the sight is tilted over to one side the less will the elevation become and the more will the gun point to the right or left of the target, according to the way the sight is inclined. In these circumstances the shell, instead of hitting the target, will fall below it and to one side. The greater the elevation on the sight and the more it is inclined, the greater will be the resulting error.

To make this clear, a vertical line, AB (Fig. 15), may be drawn on the blackboard, with A as a spot to aim at. The recruit should then be directed to aim at this spot with the sight, which must be perfectly upright, set at 1,000 yards, and afterwards to look through the rifle barrel, his attention being drawn to the fact that the axis cuts the vertical line at C above the spot aimed at. He should then be made to aim at the same spot with the same sight, _but inclined to one side_, and to look through the barrel again, when he will see that the axis, instead of being directed upon C as before, is now directed low, and to that side to which the sight is inclined, as at D; consequently as the trajectory always conforms to the movement of the axis, the bullet, instead of hitting the mark, would strike as much below D as A is below C. Draw a new vertical line through D and measure off on it a distance equal to CA; this will give the spot E, which the bullet would hit. From A draw a horizontal line AF to the new vertical line DF, then AF will show the error of direction, and FE the loss of elevation. The latter may practically be neglected; and a rule is given in Sec. =119= for correcting the former, when the small size of the target renders it necessary.

If it can be arranged it is better to show this with a gun, but a rifle barrel is usually more convenient.

=107.= _Causes affecting the accuracy of shooting._

1. With the exception of the effect of the difference of level of wheels, the various forces which act on a projectile and their effects on its flight are calculated and compensated for before the gun is issued for service.

They will not, therefore, come under the observation of the practical gunner; but there are other causes affecting the accuracy of shooting about which he must have a knowledge.

These are:--